The present invention relates to low-melting, lead-free ceramic frits.
Coatings such as vitrifiable glass decorating colors require a low-melting ceramic frit which can be applied to a substrate and fused into a ceramic coating at a temperature of about 550.degree. C. to 700.degree. C. However, such a frit formulation must also have an expansion less than 10.times.10.sup.-6 cm/cm/.degree.C. and must be at least moderately durable. Appropriate low-melting frits have an incipient fusion point less than about 450.degree. C. as measured by the dilatometer method of coefficient of thermal expansion. The durability of a glass frit can be measured by exposing glass frit powder to various attacking agents for a specified test period and at a specified temperature. For instance, a glass frit powder sample of -100/+200 mesh particle size, exposed to distilled water at 100.degree. C. for one hour and exhibiting total solubility of frit in water of less than 5% by weight, would be considered moderately durable.
It is well known in the art of ceramics that lower expansion and higher durability may generally be obtained if the melting temperature is increased. Therefore, frit materials which are low-melting yet also have lower expansion and higher durability are difficult to achieve. Conventional approaches to compounding frits which are simultaneously low-melting, lower in expansion and moderately durable have included use of lead borosilicate systems (such as those disclosed in U.S. Pat. Nos. 3,258,350, 2,642,633 and 3,404,027) or of lead zinc borosilicate systems (such as those disclosed in U.S. Pat. Nos. 3,873,330 and 3,258,350). However, these lead borosilicate systems have highly toxic lead oxide as their major constituent. This toxic lead oxide must be carefully handled during processing to avoid ingestion or inhalation.
Nontoxic, lead-free frit systems which are low-melting, moderate in expansion and moderately durable are not, however, known in the art. Some alkali borosilicate glasses are adequately low-melting, but they are not durable, and they exhibit very high expansion. Other alkali borosilicates such as those sold under the trademarks Pyrex (Corning Glass Works) and Kimax (Owens-Illnois, Inc.) are durable, low expansion glasses, but they exhibit high-melting properties. Blending or use of an admixture of such borosilicates does not, however, yield a frit having the desired low-melting, moderate expansion and moderate durability characteristics. Alkali phosphate glasses or arsenic-selenium-tellurium-antimony glasses provide adequate low-melting properties, but they exhibit such poor durability that they are soluble in water at moderate temperatures. Zinc borosilicates such as those disclosed in U.S. Pat. No. 3,113,878 may provide moderate durability and moderate expansion characteristics; however, such systems do not produce satisfactory low-melting frits.
U.S. Pat. No. 4,312,951 discloses low-melting, lead-free ceramic frits which have an incipient fusion point less than about 475.degree. C. and which are very durable. However, the coefficient of thermal expansion of these materials is generally in excess of 10.times.10.sup.-6 cm/cm/.degree.C. Consequently, for applications on glass and most ceramic substrates these materials are not suitable because coatings made with them would craze on firing.